"Street Chemistry" For Emergency Responders (Part 7)

To this point, most of the discussions of "Street Chemistry" have centered on the chemical characteristics of hazardous materials. This column will begin looking at the physical characteristics of some these materials; it could be called "Street...


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The next characteristic that affects boiling point is polarity. It causes the boiling points of polar liquids to have higher boiling points than non-polar liquids of the same or similar size. For a liquid to boil, not only does it have to overcome atmospheric pressure but you have to overcome polarity that causes the molecules to want to stick together. In order to accomplish this, more energy in the form of heat has to be applied to the liquid to get it to boil, which results in the compound having a higher boiling point.

To compare weights of compounds it is necessary to determine the atomic weights of the elements in the compounds. This information is found on the periodic table of elements. You locate the atomic weight of the element on the table and round it off to the nearest whole number. For example, carbon weighs 12.011 atomic mass units and hydrogen weighs 1.0079 atomic mass units. Carbon would be rounded to 12 and hydrogen to 1. Oxygen weighs 15.9994 atomic mass units and would be rounded to 16. Water is made up of two atoms of hydrogen and 1 atom of oxygen. As a compound, it has an atomic weight of 18 atomic mass units, one from each of the two hydrogens and 16 from the oxygen.

The average molecule of air weighs about 29 atomic mass units. So in effect water is lighter than air! How can this be? Water is a liquid at normal temperatures and pressures. It should be a gas judging by its weight.

The answer is polarity. Water is a polar compound with a boiling point of 212F at sea level. Polarity causes water, which is smaller in weight than air, to have a high boiling point and thus exist as a liquid rather than a gas. It's a good thing water is a liquid and not a gas or life as we know it might not be possible.

Of the families we have discussed in "Street Chemistry," alcohols, ketones, esters, aldehydes and organic acids are polar compounds. Hydrocarbons, aromatics, alkyl halides, nitros, peroxides and ethers are non-polar.

All materials may have some degree of polarity, but for our purposes we can make general statements about groups of hazardous materials. For example, if you compare ethyl alcohol with an atomic weight of 46 and ethyl chloride (an alkyl halide) with an atomic weight of 64, you might expect the ethyl chloride to have a higher boiling point because it weighs more. However, the boiling point of ethyl chloride is 54F and the boiling point of ethyl alcohol is much higher at 173F. The alcohol is polar and the alkyl halide is non-polar; polarity causes the boiling point of the alcohol to be higher.

A third factor that affects boiling point is isomerism or branching. The definition of an isomer is a compound that has the same formula but a different structure. When a compound is in the isomer form, it has the affect of lowering the boiling point of the compound.

One of the most important physical characteristics of a flammable liquid is its flash point. Flash point is the minimum temperature to which a liquid must be heated to produce enough vapor to allow a vapor flash to occur, if an ignition source is present (it is the vapor that burns, not the liquid).

Flash point is a measurement of the liquid temperature. Therefore, even if the ambient temperature is not at the flash point temperature of the liquid, the liquid may have been heated to its flash point by some external heat source. For example, the radiant heat from the sun, heat from a fire or heat from a chemical process may heat the liquid to its flash point. If an ignition source is present, fire can occur, and probably will. If a flammable liquid is not at its flash point temperature, combustion cannot occur. So, when researching chemicals in reference sources, the flash point temperature is the first physical characteristic responders should look for. Reference books used to research chemical characteristics in hazmat emergencies may show different flash point values.

There are different tests used to determine the flash point of a liquid. Tests use open- and closed-cup testing apparatus, which often produce somewhat different temperature values. Open-cup flash point tests try to simulate conditions of a flammable liquid in the open, such as a spill from a container onto the ground. The open-cup test usually results in a higher flash point temperature for the same flammable liquid than the closed-cup method. Responders should use the lowest flash point value given.